Forge furnace atmosphere control apparatus



March 18, 1958 J. D. NESBITT 2,827,278v

FORGE FURNACE ATMOSPHERE CONTROL APPARATUS Filed May 10, 1954 2Sheets-Sheet 2 6 L64 85 as v s 87 84C) ee 9a as J10 VOLT 101V J02 J05J04 114 107 J16 J19 mh INVENTOR.

JOHN D. NESBITT 14 BMJM ATTORNEY United States FORGE FURNACE ATMGSPHERECONTROL APPARATUS John D. Nesbitt, Sylvania, Ohio, assignor to SurfaceCon:-

blilsfiflll Corporation, Toledo, Ohio, a corporation of in ApplicationMay 10, 1954, Serial No. 428,693

7 Claims. (Cl. 266-2) This invention relates to the heating of steelforging stock to forging temperature by advancing the Work through afurnace chamber having a heating zone whose temperature is normallymaintained substantially higher than the desired leaving temperature ofthe work so that the work must be advanced through the furnace at apredetermined rate to avoid being overheated in said my invention,attention is directed to the drawing, the

following portion of the specification, and the claims appended thereto.

In the accompanying drawing forming part of this specification,

Fig. 1 is a schematic representation of the method and apparatusinvolved in the present invention.

Fig. 2 is a longitudinal vertical sectional view of the type of furnaceschematically shown in Fig. 1.

Fig. 3 is a transverse vertical section of Fig. 2.

Fig. 4 is a view similar to Fig. 3 with some modificatrons.

Fig. 5 is a detail view of apparatus for the furnace of Fig. 2.

Fig. 6 is a chart showing pertinent chemical and heat balanceinformation. a

Fig. 7 is a schematic representation of alternate apparatus foradmission of atmosphere premix gas to the furnace, which may besubstituted for a portion of the showing of Fig. 1.

Fig. 8 is a schematic electrical control diagram showing controlapparatus for the furnace.

This application is a continuation in part of my copending applicationSerial No. 98,543, filed June 11, 1949, now Patent No. 2,693,952, issuedNovember 9, 1954. This invention is well adapted for use in connectionwith the furnace disclosed in my application filed May 19, 1949, SerialNo. 94,216, new Patent No. 2,691,515, issued October 12, 1954, and willtherefore be described in that connection.

In its preferred form, the furnace chamber is cylindrical in transversecross section and is defined by a structure 5 embodying an inner annularrefractory lining 6, the structure being supported on a pair oflongitudinally extending girders 7 in a manner to permit free thermalexpansion and contraction of said structure thereon, as indicated by therollers 8, with reference to the anchored end 10 of the structure. Thecharge or work entering end of the furnace is at the left as viewed inFig. 2. In Big. 1, the furnace is diagrammatically indicated by thethree aligned rectangular figures A, B and C, respectee tively, toindicate three successive heat zones in the furnace chamber. In Fig. 1,the charge end of the furnace is at the left hand end 11 of zone A andthe discharge end is at the right hand end 12 of zone C.

If the entire work piece 13 (see Figs. 2 and 3) must be heated toforging temperature, the pieces will ordinarily be pushed through thefurnace on elevated skid rails 1 On the other hand, if the work piecesare elongated bodies 15 (see Fig. 4) of which only one end requires tobe heated, said pieces will project into the furnace through alongitudinally extending slot in the side wall thereof from anypreferred supporting and conveying means 16 outside of the furnacechamber.

Heat is produced in the furnace chamber by longitudinally extending rowsof nozzles or burners 20 arranged to fire tangentially thereinto so thatthe flame of combustion may flow along the curved surface of therefractory lining 6 whereby to constitute said lining a source ofradiant heat for heating the work pieces advancing through the furnace.Two such rows of burners will ordinarily sufl'ice.

The nozzles, or burners 20 are manifolded into groups 21, 22 and 23corresponding to the heating zones A, B and C, the respective manifoldsfor said groups are indicated at 24, 25 and 26 respectively. To minimizeexplosion hazards, each manifold has its own air and fuel mixing device27, 28 and 29 respectively. The main gas supply line is indicated at 31and the main air supply line for air under pressure is indicated at 32.Shut-off valves for these lines are indicated at 17 and 18,respectively.

Branch fuel lines 33, 34 and 35 conduct the fuel to the several mixingdevices and branch air lines 36, 37 and 33 conduct the air thereto; Eachbranch air line is provided; with a control valve 42; 43 and 44,respectively. The relative proportions of air and gas to each mixingdevice are automatically maintained by a gas governor 40in each branchgas line, the governor being responsive to changes in pressure in theassociated branch air line through a connecting pipe 41 in a manner tovary the gas flow in accordance as the air flow is varied. Ratio controlmeans of this type are Well known in the art.

The final heat zone C is not a soaking Zone where the temperature of thework pieces is equalized but is a *heating zone wherein the thermal headis maintained substantially higher, say 400 F. higher, than thedischarge temperature, say 2300 F., of the work pieces. When the demandfor heated work is at a maximum, the heat input to the zones A and B isat a maximum. When the production demand for heated Work is reduced, therate of advance of the work pieces through the furnace must becorrespondingly reduced. This presents the problem of how best tocontrol the furnace to avoid over- This problem is solved not byreducing the thermal head of the final heat zone C but by controllingthe heat input to the first heat zone A and then to the next heat zone,B to such degree as may be necessary to insure that by the time thework piece has passed through the final high temperature heat zone C, itwill not have been at scaling temperature substantially longer than whenthe work was being advanced through the furnace at maximum productionrates with maximum heat input to zone A as well as the other zones B andC. The means for effecting such heat input control to the first zone Acomprises a heat sensing element 45 mounted to be responsive to changesin temperature within the final heat zone C and more especially to thetemperature of the work pieces in said zone. The said heat sensingelement 45 operates a control instrument 46 which in turnoperates avalve adjusting means 47 associated with the heat input control valve 42for zone A to reduce the heat input to zone igsaaars A as operatingconditions may require. If still further reduction of heat input isrequired a valve adjusting means 48 associated with the control valve 43for zone B is next operated on by-the instrument 46 as will now bereadily understood. It is .preferredgto separatewzone 'A from .the nextsucceeding ZODCLB by means of=a transversally extending heat shield 49'so thatradiant heat in zone Ais localized tothatzone.; The-heat:input tozone C to maintainthe desiredelevated temperature therein is determinedby the setting of the control valve 44 for said zone. Hand setting ofsaid valve-44 will ordinarily suffice but automatic controlmeansmay beemployed.

Theapparatus thus far described is sub'stantiallythe same as thatdisclosedin' my raforesaid:application-Serial No. 94,216. g Q a i e Topermit the. already heate dxwork Tto rbeyretained in the furnacewithout-damage by overheatingrand without excessive :scale; formation:thereon in case thedemandafor heated work is;=suddenlywinterrupted,this :invention introduces into the furnace for 'contact 'withihehighly-heated r efractory lining thereof a mixture of; gases which whenheated bysaid grefractory will react endothermically and thereby rapidlylower the :temperature of the surface of said refractory; the saidmixt'ureof gases (hereinafterjsometimes' called .a'tmosphere rgaspremix) being of such -character'ithat the reaction productswill'serveas a protective atmosphere for the-heated -work to reduce theformation of'scale thereon to a.-minimum. The atmosphere gas premix is'formed according to Fig; 1, outside of the furnacechamber in amixing'device 5l to which air underg'appropriate pressure is deliveredbya'supply'pipe-52 havinga control :valve 53 and, to which fuel gasunder appropriate. pressure'is delivered by'a" supply pipe 54;.havi ngacontrolvalve 55. The relative proportions-of the airiand'fuelxgas' thusdelivered to said mixing device =51 are .eontrolled in any preferredway. -Sirice the work in: zone C: is already at elevated temperature andthereforerequires; immediate protection against overheating, ;itis:-into:zone;C that the mixture from the mixing device 51: is preferablyintroduced whereby to rapidly cool the refractoryilining of that zoneandto supply a protective'atmosph'ere: to surround the hot work'in'thatzone. The mixing z-devicesl therefore delivers to the manifold 26 forthe: burner group-23 associated with the zone C so thatihemixture mayenter said zone tangentially andthereforewbe rapidly heated to reactiontemperature ,4 actual closing of the furnace chamber and inlet ofatmosphere premix gas for safety reasons.

In Fig. 2 the flue dampers 67 and 68 are shown as hingedly connected toan edge of the respective flues.

its secondposition andthe cylinder -is thus actuated tois illustratedin. Fig. '7,'-wherehighipressure.plant-air,

boundary slirfaceiofisaid zone. To reduce draft through .the'furnace'chamber to a minim'um when the fuelsupply .to'the furnace is .shut 151T,thenormally open flue dampers 67 and 68 and'the door E71 are-tlosedfitbeing understood that it is; desirable to m'ai'ntain a plenum' ofprotective atmosphere in the'furn'ace chamber d'uringthat tim'eto reducethe formation ofi-scaleon the'h'eated'w'o'rk to a minimum. a

The loss of he'at'fromQthe' furnace. through its walls, its water-cooledwork support rails 14; and the application of the atmosphere gas premixrather quickly cools' the furnace chamber to atemperature b'el'ow whichthework is not. overheated, and by subst-ant-ially simultaneouslyclosing the furnace "chamber and generating a'-worl protectingatmosphere within the-"furna'ce," a relatively small: quantity ofatmosphere gascan efiectively reduce and substantially eliminate airi-nfiltration into the furnace chamber andconsequent oxidation'of thesurface of the workduring'suchtemporary delays. v 7

While a simultaneous shutting on of the firing system, starting of theatmosphere inlet system afid..closing -.of the furnace i'cha'inber' .is"desirable for fullest benefit from this atmosphere protectionlandavoidanceofovrheating of the'work, in practiceashort delay"after'stoppingthe firing and closingthefurnaee chamberis desirable toinsure purging or the.- burner manifold piping randithuszto avoidbackfire. i'It: is thus preferred to .Zsii'ghtly: delay ward "theposition shown in dashed lines, moving the damper 68 from thebpjempositiomasshown to a closed position shown in dashed lines atop theflue 81. If desired, the flue dampers 67, 68 and the door 71 may beinternally. cooled by streams of; coolant:waterzisupplied through"flexible ;conduit s,,.notshown.

--When-the size of; the :piping-jand-Nahesyinfithe: cornbustioneairand'fuel lines becomes.-so:zlarge::that. it.:is diflicult :toautomatically:close theqair line's,-.it may be preferred-to. stop {the combustiomair'blower' 82 and provide an .-.-ind ependent-source, of. air .-foratmosphere. This is particularly. advantageous where power interruptions-may be experiencedand a- -highpressure "plant. air storage systemisavailable. iSuc'h:an atmosphere system perhaps'up 405150poundsspersquareFinch; is delivered toair supply:conduitfiiythroughsolenoid valve 84- and toan inspiratori-SS wherein-atmosphericrain=isinspirated and delivered to a'mi-xer86atiabout 30p. 's; i. Fueltgas suchas natural gas is delivered-from fuel-supply conduit 87 through solenoidvalve: 88;" pressnrez governor: 91 and to-the-mixer 8,6. Thegove'r noris 'backloadedtby aesm'all conduit92 :to the mixt urezconduiti93 idownstream of the mixer 86, to maintain zero gas pressure at the throatof the mixer; 86 which-is preferably?of-the'ventnri type as is wellknown, thus providing the desired proportioning' of fuel and air forproducing-an atmosphere-insthe furnace chamber. The inixtur'ein'n:conduit1-93 passes through mixture valve94'and inletpipe-95Zto anozzlea96 forthe discharge zone? of the. furnace; 'zone .C in' Figs. 1and? 2. A second stream;ofrmixtiirqor'atrnosphere premix, may pass?throughdnlet-xpipe: 99 and yak e 97 ?'therein, 1 and inlet nozzle 98into zone B of the furnace. 'By reducing ThGZ'IIHmbELT ofatinospirereiprfemixnozzles, a" highereinlet velocity is obtainable,hence improved wall contactl is obtained, *andiits'is notrithennecessaryato utilize as' many inlet nozzles as are used for combustionpremix ga's. :1t maybe-desirable ,tonse a;:plurality'of'iannospherepremix nozzlesr 96 .:and-98,:-but:general1y With=.30.p.-s. ikin'th'epremix line, one oritwo;.-'each= of such nozzles: will suifice providedthe furnace chamber is sufficiently closed asby doors andflue dampers.

An electrical controlsystem forcoordinating the illustrated atmosphere;producing apparatus with the furnace operations "isrillus'tfate'd in'Fig'.' 8 wherein-a volt control .circuitris illustrated. A two positionatmosphere push button lillgshown in'oif, orropen, position is wired inseries with: a safety? contact 102 ina relay operated from a contactinEthe temperature control instrument, not-shown andtthessolenoid coil-1'03 of a relay 104. The relay operating contactin the temperaturecontr'ol instrument" isisetrto' iclose'i the relay abov'e' 1400 Fa andto ope'n itrhe1ow'::1400 F.- so -thatati iosphere remix cannot:berdelivered to the furnacewhjen it iscolder'than 1400".F.,.:thus'eliminating-an explosion hazard. The V safetyi'contact 102. is; shown:in its safe;--er hot= position, closed;

remap-104 :operat'es 'tliree contacts 105, 106 'ind' 1'07. :Sontact IOSisi normaIiy closed a'nd --1s =-1ni-series witlfa normallyi open-contact-108in a limit-switch 111' which isi d'epressed when the damper: 68 i'sin openpositiom aiid .the 'oper'ating c'oil 112 of an 'airblower Mayne.*Th us a-samra the combustion air blower 82 cannot be started oroperated unless the damper 68 is in open position, off the fine 81. Whenthe push button 101 is depresed to actuate the atmosphere producingcontrol system, the combustion air blower 82 is stopped.

Contact 106 operated by the relay 104 is normally closed and is inseries with a coil 114 in the main fuel gas valve 31, hence when theatmosphere push button 101 is depressed, relay 104 is actuated andcontact 106 is opened, thus breaking the circuit to the coil 114 of themain gas valve 31 and shutting down the nozzles which supply combustionpremix to the furnace chamber for heating the same.

Contact 107 is operated by the relay 104 and is normally open. When therelay 104 is actuated, contact 107 is closed, making a circuit to a coil116 in a timer 115. The timer 115 is adjusted to run for a short periodof time such as 30 seconds, sufiicient to allow the furnace chamber topurge itself and the residual air in the combustion air piping to drawthe last bit of fuel gas from the gas manifolds. When the timer 115trips out, it closes a normally open contact 117 therein, which is inseries with a contact 118 which is closed when the pushbutton 101 isdepressed, thus making a circuit to solenoids 78 and 122. Solenoid 78operates the four-way hydraulic valve 77 from its position to open thedamper 68 as shown to its second position to close the damper 68. Asimilar solenoid and hydraulic valve, not shown, is also actuated byclosing of contacts 117 and 118 to close the other flue damper 67.Solenoid 122 operates a similar four-way hydraulic valve for a dooroperating bydraulic cylinder, not shown to close the discharge door 71of the furnace simultaneously with the closing of the flue dampers.

While the timer 115 is preferably a clock mechanism, especially wherecycle electric current is used and upon stopping of the power to the airblower, the blower air pressure drops rapidly, it is equallysatisfactory to use other timing means. For example, when the blower 31is driven by a 60 cycle current motor which characteristically coastsafter stoppage of power supply, a pressure switch may be placed in themain air manifold 38 and adjusted to close a contact 117 therein after adrop in air pressure therein to a pressure attained in about seconds.

When the solenoid 78 is actuated and the flue damper 68 is moved fromits open position off the flue as shown in solid lines in Fig. 6 to theposition shown in dashed lines in Fig. 6, a limit switch 121 isdepressed, closing a normally open contact 123 therein. This makes acircuit through a contact 124 which, like contact 102, is closed above1400 F., in the temperature control instrument, and energizes a solenoid125 for the atmosphere gas valve 83 in pipe 87 and solenoid 126 fortheatmosphere air valve 84 in the auxiliary air supply pipe 83. Ifdesired, the mixture valve 94 may also be operated by a solenoid wiredin parallel with solenoids 125 and 126.

The above described control circuit is designed to coordinate theclosing of the heating chamber by flue dampers and suitable doors, wheredesirable or necessary, and to provide a time delay between the stoppingof the firing system for heating the furnace and the closing of thefurnace chamber. This allows the furnace chamber and fuel supply pipesto be completely purged of unburned fuel before the atmosphere gaspremix is admitted thereto, and automatically opens the atmospherepremix, or air and gas, valves upon closing of the furnace, thuscontaining the generated atmosphere within the heating chamher andreducing the quantity thereof required to protect the surface of thework therein.

The reaction produced by scrubbing the atmosphere premix on the Wall ofthe furnace chamber is an endothermic reaction since more heat isrequired to bring the gas to furnace temperature than is supplied fromthe partial combustion reaction which generates the atmosphere, but agreater cooling efiect from the water cooled rails 14 and from normalheat loss from the furnace walls is obtained thus reducing the time thework which is temporarily retained in the furnace is held above scalingtemperature.

Figure 5 illustrates some chemical and thermal properties of varyingratios of the atmosphere gas premix. The ordinate is the ratio of air tomethane and the abscissa is the reacting or equilibrium temperature.Curve 61 shows the thermal equilibrium mixtures of air and methane atvarious temperatures. Mixtures falling below the curve at a giventemperature will react endothermically, and the proportionate distanceof a point from the curve is an indication of the quantity of heatabsorbed in the reaction. Curve 62 shows the temperatures at which thegases resulting from the reaction to equilibrium of a gas mixture of theair to gas ratio indicated will be in equilibrium with FeO, any richerpremix resulting in an atmosphere reducing to FeO, and a leaner premixresulting in an atmosphere oxidizing the FeO. Curve 63 shows the sameinformation with regard to Fe O and the point 64 lies on a similar curvefor Fe O In the application of the present invention an atmosphere gaspremix is chosen that is reducing to the several forms of scale, thusconstituting a protective atmosphere, and at the same time having a highcapacity to absorb heat in an endothermic reaction. The preferred ratiois 1:1 or /2 :1 air to methane gas, thus allowing a maximum coolingeffect without the bad effects of carbon deposition or sooting characteristic of richer airzgas mixtures. The mixture is admitted to therefractory wall in relatively small or thin streams for improved heatcontact therewith for suflicient time to insure substantially completereaction before passing to the center of the furnace, thereby protectingthe work from further oxidation while cooling the furnace and the workto a desired temperature.

As an illustration of the beneficial effects obtained in the practice ofthe present invention the following may be cited:

Work was heated, in a furnace whose walls were maintained at 2700 F., toa core temperature of 2200 F. The scale formed in normal operation at50% of the furnace design production rate was 0.010 inch on a 23 lb;piece of work, and after holding in an atmosphere in accordance withthis invention for 25 minutes the scale was 0.008 inch. After reheatinga corresponding piece to forging temperature, the scale was 0.017 inch.A similar work piece was heated to forging temperature, then coolednormally without an atmosphere to 2l00 furnace temperature in 30 minutesand was found to have 0.56" scale.

It is preferred to admit the atmosphere forming gas to the furnacechamber through a series of laterally extending relatively small inletports tangent to the inner surface of said chamber whereby to produce inthe chamber a plurality of laterally spaced streams of said gas andwhich collectively form a relatively thin layer of the gas next to saidsurface with the resultant more rapid cooling of said surface byendothermic reaction as will now be readily understood.

Since the preferred type of high heat head forging furnace illustratedin Fig. l utilizes a series of laterally extending burner ports whichare tangent to the refractory wall, and those burners, or nozzles, areturned off when the work is temporarily retained in the furnace, thesame ports 20 may be used to deliver the atmosphere gas premix to thefurnace. It is sometimes preferred, however, to utilize an independentatmosphere premix inlet system such as shown in Fig. 7, and in eithercase the atmosphere premix inlet system is coordinated with the damperand/or door controls as hereinbefore described.

I claim:

1. In a furnace for heating work to elevated temperatures, thecombination which comprises structure delining a cylindrical furnacechamber having refractory walls wherethrough work may be advanced from acharge e'nd't'o a dischar'ge end thereof-fjror heating the work; saidstructure defining ia't least-'oneflue intermediatefsaid endsfondis'charging products of combustion from the furnace chamber,=da'm'p'er meansforclos'ing said flue,-'"bur'ner means for firingjntosaid chamber to heat the refractory "walls thereof to- :onstitute thesame 'a sourc'e of radiant heat tending tojoverheat the workgfbnrnerstopmeans for shutting ofi the burner -mea'ns, atmosphere means foradmitting tothe fnrnace ehambe'r next adjacent the'refracmry'wallthereof=a -streain of gaseonsinixture which will be heated to reactiontemperature by eontact with said WaH,*coo}-the-'wall, and 'gene'ratea-protective atmosphere for-preventing oxidation'of the work in thefnrnacebhamber-andcontrol 'rneans *co'nn'ec'ted -to-said damper means,

said -burnerstop 'rneansyand sa'id atmosphere =means operable tofloperate the'bnrner stop meansyactnate the damper damper controlcircuitito-rcause theiflue damper to :close after: aldelay periodsetbyithe timer relay, and a .control circuit 'for actuating theatmosphere means after therfiue damper xisclosed tocansesthegaseousmixtureto be admitted toithejurnace after said delay. period.

2. In the combination-according toclaim 1,:at.-least one door -=for'iclosingian::end:of the Efurnace ,fchamber; and means comprisingarcontrol, circuit :for closing s'aids door, the timer contact meansbeing adapted to actuate :the door closing meansxthrough 2 its ;-contro1circuit to close 7 the door aftercsaid delay;period.

3. 'In a furnace -for,heating ,work to; elevated temperature andcomprising wall means forming a heatingichambert 'wherethrongh workymay-be= -;adjvanced for heating, and heating means ifornormally-maintaining.said-wall means; :at an; elevated temperaturetending: to overheat the Work, theimprovement which comprises atmospheremeans. for supplying "an atmosphere: premix; gas ,to the heatingchamber, closure means :forclosing the-heating chamber, and coordinatingmeans connectedto thheating means,;the closure means,; and;theatmospheremeans, for substantially simultaneously shutting offitheheating means, actuating the closure means to close-the heating.cha-m'her, and actuating the 'atmospheremeansto deliver atmospherepremix gas to the heating chamber. a

'4. A furnace according to claim '3 vvherein the coordinating meanscomprises time ,delay means for-delaying the closure of the heatingchamber :after :the heating means is shut ofi.

7. Ina furnacei for heating tworkw to ameleva ted .tem-

pera'ture andaromprisingmll inreans forming .la heating chamberwherethrough worksmay beaadvanced" for. heating, andslheating means for:norma11y-,-Jmaintaining.isaid wall means at an elevated temperature:tending to overheat the work, itheiimprovementswhichzcornpriseso'atmosphere-meanstfor supplying amatmosphere' premix gas FtOthe heatinge chamber-1 :40 prevent. 'oxidationl not the. work when it;is iretainedan'n sthe heating chamber temporarily, closure,means:,-for;closingthe ,rheating chamber,.-and ,a control Icircuit:connectedzL-to,.v:,.said; :heating means, ,said atmosphere zmeansnndjisaid closnrejme.ans,-,; said ,control circuit includingia:source'tof voltage aseriesrcircuit including a =switch;-.-.and 5a.;first. zrelayecoil connected across saidiz source ansecondl series:circuit; including a normallyclosedfiont 'c t.--.-of; said1firstrrelay 0ndlan 0n ro1-.- or operating said heating means conrrectedacrosssaid -so urc e, azthird,scrieseircuinincluding ainq ma yp n ont t: ofsaidafirst-ir'cla i amthe coihnf ait m de ayay cennci tcdzacrosswsa ds os n fou hs- 1i s udin am rmal nop nnta tz 0sa d m 1 1 y relay nd.- cmro1opera ls rl n elmea r n e across 1d onrce, and 'a fiflth seriescircuit-including sw tch n e pon iv a s-operatio fl-Sa c r me ns d, amnlrp c pperatin zsai a mosp q e ea connected across said source, saidcontrol circuitoperatin inlrespa se to Qpma Qnpfis d; tamen i i oppensai i1 m YiQ Q snma tw fls si t re ay QQi ma ii hfififibli#QK QR KUM izh tiname sna d, after,at time, delay, tmoperategsard closure means toclose e -hi at g g: chamb ans x-a t fr-gcl of ;h a n h mh t opera e-s ia mo p e e-mea to supr y saigle s smt e h t n sham er.

A i t rn cs t rh h gfilefl f h sp te 7 i UN EB TEAI PATEN 1,783,156'Ijalley Nov, 2 5, 19 30 1,937,017 Greeri -'N0V; 28,1 933 2,233,474 f-Drefiein 'Mar. 4, '1'941 2,237,785 x Knapp :Apr; 8, 1941 "22,420,415 BriSto1 May 13,;1947 1' 2,628,830 Krr Feb;- "17, '1953 2,693,952 iNe-sbitt':Nov. 9, :1954 2,763 f476 Ness 'et-* al.- Sept.-' l'8; 1956 OTHER;REFERENCES

